Photovoltaic power systems are undergoing rapid development as primary power sources for space as well as for terrestial uses. The early space shuttle flights will utilize a power extension pack (PEP) solar cell array to provide approximately 32 kilowatts of electrical power from over seventy thousand large area silicon solar cells. This solar cell array will measure approximately 240 feet in length and will be capable of being retrieved and returned to Earth between missions. Domestically, arrays producing up to one-half megawatt of electrical power are being produced. Further, in addition to these and other large power projects, there are many other specialized applications wherein there are unique power requirements. Thus, there exists a serious need for solar cell constructions which provides a high voltage output and can serve pressing demands both on the Earth and in space.
Early work in this field began with the "solar battery" developed by Bell Telephone Laboratories in 1954. (See Chapin et al, "Bell Solar Battery", Bell Laboratories Record, Vol. 37, 1955, pp. 241 et seq.). This battery or cell basically comprised a "wraparound" junction device wherein the base was covered with a thin diffused layer over its entire surface except for a circular area etched on the back of the cell. Ohmic contacts were made to the respective "n" and "p" regions. This "wraparound" device evolved into a rectangular planar form with a diffused surface having a metal grid collector on the top side and a metal pad collector on the bottom. This form of cell is still the most commonly used for space solar cells. Reference is made to Matlow et al, "Ohmic Aluminum-n-Type Silicon Contact", Journal of Applied Physics, Volume 30, April 1959, pp. 541-543 for a discussion of such cells.
In the 1960's, a photovoltaic device, the thermophotovoltaic (TPV) cell, was developed which converted a narrow portion of the thermal spectrum into electrical energy. The device utilized an interdigitated contact structure covering the back of a germanium cell wafer. Because the device utilized narrow band heat radiation, photons are absorbed uniformly throughout the bulk of the wafer, and thus this type of back contact was satisfactory. However, for most visible light conversion, wherein most of the light is absorbed close to the surface and wherein migration of the carriers is effected by diffusion to the collectors, such a back contact arrangement requires several concessions. For example, the device must be thin and must be made of high quality semiconductor material having a very high lifetime. The TPV cell evolved into the interdigitated back contact silicon solar cell.
In more recent times, a modification of the coplanar back contact solar cell was developed which is known as the Tandem Junction (TJC) solar cell. This device relies on a front surface field region in order to lower surface recombination losses and to provide an electrostatic field region which assists in sending the carriers to the rear surface of the cell for collection. Reference is made to Chiang et al, "Thin Tandem Junction Solar Cell", Conference Record, 13th IEEE Photovoltaic Specialists Conference, IEEE, Inc., New York, 1978, pp. 1290-1293 and to U.S. Pat. No. 4,113,698 (Chiang et al) for a further discussion of TJC cells. The subject matter of these materials is hereby incorporated by reference. Other important developments include Vertical Multijunction (VMJ) solar cell (see Gorandia et al, IEEE Trans. on Electron Devices, Vol. ED-24, p. 342, April 1977), the Horizontal Multijunction (HMJ) solar cell (see U.S. Pat. No. 3,994,012 (Warner)), and the V-Groove Multijunction (VGMJ) solar cell (see Chappell, IEEE Trans. on Electron Devices, Vol. ED-26, p. 1,091, July 1979).
In general, all of the more recently developed devices, including the IBC, TJC, VMJ, HMJ, and VGMJ solar cells, represent improvements over the prior art particularly with respect to reduction or elimination of grid-shadowing and the sheet resistance component of series resistance. Further, all of the cells except for the IBC and TJC cells, operate as high-voltage low-current devices and thus provide a reduction in series resistance as well as in temperature effects due to I.sup.2 R heating. In addition, significant transparency to infrared photons further reduces high temperature effects in these cells.
Other patents of possible interest in this field include U.S. Pat. Nos. 3,150,990 (Rudenberg); 3,278,337 (Gault) 3,969,746 (Kendall et al); 3,982,964 (Lindmayer et al); 4,052,228 (Russell); 4,072,541 (Meulenberg, Jr. et al); 4,099,199 (Wittry); 4,101,351 (Shah et al); 4,128,733 (Fraas et al); 4,131,486 (Brandhorst, Jr.); 4,135,950 (Rittner); 4,160,678 (Jain et al); 4,166,919 (Carlson); 4,167,015 (Hanak); 4,217,633 (Evans, Jr.).